“So if we can isolate irritability we can perhaps develop treatments that are best targeted towards people that are particularly irritable.

"Somebody could be very depressed but have no symptoms of irritability — on the other hand, someone could have depression and the prominent emotion may be irritability more than sadness. And you’d likely want to treat those two people in different ways.”

The good news is that Holtzman believes short term irritability is largely down to a lack of sleep. So if you're feeling cranky, get an early night and you may be smiling in no time.